1. Field of the Invention
The present invention relates to a power-variable finder optical system, and, more particularly, to a Keplerian zoom finder optical system including a power-variable zoom objective lens.
2. Related Background Art
Finder optical systems are classified into the inverted Galilean virtual image finder, with which a virtual image enlarged by an objective lens can be observed, and the Keplerian finder, with which an intermediate image formed by an objective lens and enlarged by an ocular can be observed.
The virtual image finder exhibits a simple structure and a reduced cost. However, it involves the disadvantage of an unclear boundary of the visual field thereof.
On the other hand, although the Keplerian finder is disadvantageous of the slight complexity of its structure, it is advantageous in function since the visual field can be clarified by disposing a field diaphragm on the focal plane of the objective lens thereof.
In recent years, camera finders of a type capable of continuously varying in their variable power have been required during the progressive improvement of multi focal points imaging lenses for lens shutter type cameras. A variety of disclosures have been made about the virtual image zoom finder. For example, the disclosure disclosed by the applicant of the present invention in Japanese Patent Laid-Open No. 63-148228 achieved a sufficiently wide visual angle of 54.degree. at the wide angle end thereof and an excellent variable power of 2.5 to 3.0 times.
Some disclosures have also been made on the Keplerian zoom finder as disclosed in Japanese Laid-Open No. 61-156018, Japanese Patent Laid-Open No. 62-7017, and the like.
However, the conventional Keplerian zoom finder suffers from a relatively narrow visual field of about 50.degree. at the wide angle end thereof and poor variable power of two times or less. Therefore, the conventional Keplerian zoom finder is not used often since it cannot keep abreast of the trend for a winder angle in the imaging lens and a higher variable power.
In the Keplerian finder, the objective lens thereof and the ocular thereof must be individually subjected to aberration correction in order to make the visual field clear. Therefore, although a conventional imaging zoom lens may be utilized as the zoom objective lens for the Keplerian zoom finder, the complicated structure of the imaging zoom lens renders it unipreferable use as the zoom objective lens for a finder which must be simplified in structure and be reduced in size.